Removal of deleterious gases from molten metal



United States Patent 3,205,067 REMOVAL OF DELETERIOUS GASES FROM MOLTENMETAL Charles Henry King, London, England, assignor to The BritishOxygen Company Limited, a British company No Drawing. Filed Mar. 13,1963, Ser. No. 264,767 Claims priority, application Great Britain, Mar.22, 1962, 11,095/ 62 2 Claims. (CI. 75-59) In the manufacture of steel,it sometimes happens that trouble is experienced in the finishedproducts due to the presence of excessive amounts of hydrogen gas whichhas become dissolved in the metal during the melting operation. Whenhydrogen is present in amounts above the safe limit, porosity occurs inthe ingots :or castings, and in the case of the former, flaking, hairline cracks and other forms of unsoundness may develop in subsequentprocessing even though the porosity was not obvious in the ingot form.Porosity also arises from this cause in the preparation of ingots orcastings made from non-ferrous metals, and in iron castings.

Methods which have so far been employed for reducing excessive amountsof hydrogen in metal include bubbling an inert or relatively inert gasthrough the molten metal, or blowing the surface of the metal with sucha gas. Examples of gases used in these methods are argon, helium,nitrogen, carbon monoxide and carbon dioxide, selection from this rangedepending on the type of metal treated. Alternative methods for reducingexcessive hydrogen content are vacuum casting and prolonged heattreatment of the solidified metal. All these methods have thedisadvantage of requiring special apparatus, which in the case of vacuumcasting and of heat treatment, is particularly costly. In the case oftreatment with inert gases, the main disadvantages are the additionaltime required during which the metal must be maintained in the moltenstate, the consequent loss of temperature, and the splashing which mayresult from the treatment.

A further method which has been employed, particularly with ferrousmetals, is the use of a solid compound which, when placed on, or plungedbelow, the metal surface, decomposes to produce a gas which issubstantially insoluble in the metal but into which the hydrogen candiffuse and thus be removed. Normally the type .of material used forthis purpose in the steel industry is a carbonate, such as limestone,which on heating decomposes to give carbon dioxide gas and an oxide,such as calcium oxide (lime). In steel degassing operations it isdesirable that slag, particularly lime based slags, should be absent,and it is a disadvantage of the method just described that a solidmaterial such as lime is a product of the reaction. When limestone isadded to the surface, a layer of lime or slag forms in the vicinity ofthe added limestone ice creating a barrier between the metal surface andthe carbon dioxide gas, thereby reducing the efficiency of trans fer ofhydrogen into the gas. Also there are operational disadvantagesassociated with plunging solids below the surface of molten metal.

According to the present invention a process for reducing the hydrogencontent of molten steel comprises introducin g to the molten metalsurface an inert or relatively inert gas in the liquefied state. It hasbeen found by analysis that when gas is added in this form, asubstantial amount of hydrogen is removed; this is accomplished withoutexcessive loss of heat and without the production of a slag. This istrue even though the temperature of the gases in these states are verylow, e.g. liquid nitrogen, l98 C. and liquid argon, 184 C.

The liquefied gas may be added to the molten metal in any convenientmanner which insures that the bulk of the material reaches the surfacein the liquid state.

A feature of the process is that, on the liquefied gas reaching thesurface, the instantaneous change from the liquid state provides ablanket of gas through which the transfer of heat is sufiicientlyretarded to enable the liquefied gas to move to other areas andeventually provide a thorough scavenging of the metal surface.

The invention is illustrated by the following experiment:

100 lbs. of scrap was melted in an electric furnace, under conditionsconducive to obtaining a molten metal with a high hydrogen content. Thebulk of the slag was removed from the metal surface, and a sample of themolten metal removed for analysis. About 1 /2 lbs. of liquid nitrogenwas carefully added to the molten metal, and, after two minutes, afurther sample of metal was taken for analysis. During this time thefurnace currrent was passing, no ill effects being observed. It wasfound that the first sample taken contained 3.8 cc. of hydrogen per 100grams of metal, and the second sample contained 1.7 cc. of hydrogen per100 grams of metal, indicating a substantial degree .of degassing.

I claim:

1. In the process for reducing the hydrogen content of molten steel bythe action of nitrogen thereon, the step of introducing the nitrogen inits liquid state to the surface of the molten steel.

2. The process as claimed in claim 1, wherein the proportion of liquidnitrogen to molten steel is of the order of one and a half percent byweight.

References Cited by the Examiner UNITED STATES PATENTS 2,068,785 1/37Bain et al -59 2,395,458 2/46 Cape 7559 3,042,484 7/62 Mating 75 59BENJAMIN HENKIN, Primary Examiner.

1. IN THE PROCESS FOR REDUCING THE HYDROGEN CONTENT OF MOLTEN STEEL BYTHE ACTION OF NITROGEN THEREON, THE STEP OF INTRODUCING THE NITROGEN INITS LIQUID STATE TO THE SURFACE OF THE MOLTEN STEEL.